Help: Question on biomechanics

I know that all body positions and movements (such as standing,
push-up, pull-up, sitting etc.) are based on principles of physical
levers (1,2 and 3 classes). However sometimes it is not easy for me to
determine what class of lever a body position or movement is represent.
Please help me to understand the general rule (or rules) which I could
apply for resolution of my problem.
Thank you.
Boris boris7 at ix.netcom.com